I used clipless all last year with a mtb shoe that's a size too big for me. I wore a thin poly sock, a thicker wool sock, and then a gortex/waterproof sock. I rode in temps as low as -2F. I can't say my feet were warm, but I still have all my toes.

This year I'm going to get shoe covers and see how that works.

Never thought of adding an insole... that would put some insulation between my foot and the bottom of the shoe. That's a great idea.